The one-mind which manifests either as 'unen' [thought-good and evils and all human emotions] or 'munen' [no-thought, no-emotions-not good, not evil] must be something which is beyond these conditions. It must be the light which illuminates everywhere and is never clouded. As soon as you become clearly aware of this light, you will be released from the limitation of delusory thoughts, and the Buddha's wisdom will be realized.
...
The Third Patriarch said, "When the cloudless light illumminates itself, there is no need to make mental struggle, there is no waste of energy." This is the vital point of the practice and enlightenment of this samadhi. "The cloudless light illuminates itself" means the light of the [True] Self shines brightly. "Not to make mental struggle" means not to add the illusory mind's discrimination to the reality. When you make mental struggle, the light becomes illusory mind and brightness becomes darkness. If you do not make mental struggle, the darkness itself becomes the Self illumination of the light. ... For example, it is like the light of the sun or the moon illuminating everything-mountains and rivers, human beings and dogs, etc. equally, without differentiation or evaluation. Also, a mirror reflects everything without bothering to discriminate.
...
When our mind is good, we become stiffened by good intention and attached to the limited results of the three good realms. Consequently, we become blind to the light which is beyond goodness. When our mind is evil, we become stiffened by evil intention and pulled by the results of the three evil realms [greed, anger, ignorance]. Consequently, we suffer and cannot be aware of the light which is beyond evil. When we are in the condition of no-thought, we stagnate there because we think it a desirable stage of mind. ... We fail to realize the light beyond no-thought. When we transcend the dichotomies of good and evil, thought and no-thought, and emit the light of the Self, settling beyond discrimination, we will not stagnate in goodness though our mind be good. Nor will we attach ourselves to evil, or to the stage of no-thought even though our mind be in that condition. Therefore, even when our mind becomes evil, if the light of beyond-thought is emitted, evil mind will be dropped off immediately, and there will be only the light of the [True] Self. This is the way to lead people in the three evil realms to annuttara-sammyak-sambodhi (ultimate awareness). The same occurs to the people dwelling in the three good realms too, and allows them to step over into ultimate awareness.
***
Only when we sit zazen in our daily life, are our eyes opened to the reality outside of the domain of thoughts and discrimination. We just illuminate our thoughts which moment by moment come up and go away, refrain from fabricating adoption or rejection, and hatred or love. As with the Tathagata, what we do in our zazen is expressed as radiating the great light, illuminating the whole world of the ten directions, and releasing all sentient beings from suffering.